This invention relates to a catalyst precursor composition and to an improved process for the production of linear alpha-olefins by catalytic oligomerization of ethylene using a catalyst obtained from the novel precursor composition. More particularly, this invention is directed to a stable complex of nickel, ethylene and hydride in diol solvent and to an ethylene oligomerization process in diol solvent employing a nickel complex catalyst composition formed by adding the pre-formed stable nickel complex in diol solvent and a bidentate ligand in separate portions to the oligomerization reaction zone.
Linear monoolefins are compounds of established utility in a variety of applications. Terminal linear monoolefins, particularly those having 12 to 20 carbon atoms per molecule, are known to be useful as intermediates in the production of various types of detergents e.g. alcohols, ethoxylates, etc.
Several synthetic techniques have been developed for the preparation of terminal linear monoolefins in the detergent range. One very attractive synthetic method from the standpoint of raw material availability and cost involves oligomerization of ethylene to higher molecular weight linear monoolefins (even numbered alpha-monolefins) by contact with a catalytically active nickel complex dissolved in certain polar solvents. One class of suitable nickel complex catalysts for ethylene oligomerization is prepared as the reaction product of an olefinic nickel compound, including zero-valent nickel compounds such as bis(cyclooctadiene) nickel (0) or -allyl nickel compounds, and a suitable bidentate ligand as described in U.S. Pat. No. 3,644,564 to Van Zwet et al, U.S. Pat. No. 3,647,914 to Glockner et al and U.S. Pat. No. 3,647,915 to Bauer et al. A different and preferred class of nickel complex catalysts can be prepared by contacting in certain polar organic solvents in the presence of ethylene (1) a simple divalent nickel salt which is at least somewhat soluble in the solvent, (2) a boron hydride reducing agent and (3) a suitable bidentate ligand. The preparation of catalysts in this preferred class and their use in ethylene oligomerization are described in U.S. Pat. Nos. 3,676,523, 3,686,351 and 3,737,475 to R. F. Mason and U.S. Pat. No. 3,825,615 to Lutz. PG,3
In the above mentioned patents describing ethylene oligomerization with the preferred nickel complex catalysts, it is taught that the catalyst composition is suitably preformed outside the oligomerization reaction zone by mixing together in the presence of ethylene, the various ingredients--i.e., the nickel salt, the bidentate ligand and the boron hydride reducing agent--in the polar organic solvent. This preformed catalytic composition in the polar organic solvent or diluent is then added directly to the reaction zone. In this regard, no real criticality is attached to the order or manner in which the catalyst precursors are combined, in the patent teachings, although it is commonly preferred to contact the solvent, the nickel salt and the bidentate ligand in the presence of ethylene before the boron hydride reducing agent is added to the solution of catalyst precursors.
While the preferred nickel complex catalysts prepared according to the teachings of the aforementioned patents to R. F. Mason and to Lutz provide an attractive means for oligomerizing ethylene to higher linear terminal olefins, in particular those in the detergent range, the resulting oligomerization processes are not devoid of problems. One troublesome aspect of these preferred processes is the propensity of the nickel catalysts to catalyze the formation of objectionable, polymeric polyethylene under certain conditions in the process. This polymeric polyethylene typically has a broad molecular weight range (molecular weights from a few thousand to as high as a few million) in contrast to the desired lower molecular weight oligomer product. As produced in the oligomerization process, such polyethylene is not a usable commercial product and thus only serves to decrease the yield of desired oligomer product from the ethylene feed. Furthermore, it has an even more objectionable effect in that it tends to plug and foul mechanical equipment and transfer lines in the process.
The formation of this polymeric polyethylene in the oligomerization reaction product and its objectionable effect on downstream processing equipment is recognized in U.S. Pat. No. 4,020,121 to Kister et al. Specifically, this patent teaches that residual catalyst present in the hydrocarbon (oligomer) phase of the three phase oligomerization reaction product (the other phases being a liquid solvent phase and a gaseous ethylene phase) can promote the formation of polymeric polyethylene when catalyst, solvent and ethylene are present in the hydrocarbon phase at conditions under which part of the hydrocarbon phase is removed by flashing or distillation. According to Kister et al, this polyethylene formation downstream of the oligomerization reaction can be avoided by a stepwise product recovery process in which the hydrocarbon product phase is subject to a scrubbing step using additional liquid reaction solvent prior to the time that the catalyst-contaminated hydrocarbon product phase is subjected to depressurization for removal of ethylene.
A second troublesome source of polymeric polyethylene which is not dealt with in the Kistar et al patent is the portion of the oligomerization process which is upstream of the oligomerization reaction itself, that is, the reaction vessel (catalyst maker) used for making the preformed oligomerization catalyst and associated reactant transfer lines into the catalyst maker and out of the catalyst maker to the oligomerization reactor. Here, it is found, particularly when the catalyst preparation is carried out in a continuous fashion using an aliphatic diol reaction solvent, that polymeric polyethylene tends to form and periodically plug off the transfer line from the catalyst maker to the oligomerization reaction zone and the transfer line used to add makeup ethylene to the catalyst maker. Further during periods of minor upset, significant quantities of polymeric polyethylene may form in the catalyst maker itself necessitating shutdown and cleanout operations. When it forms in the catalyst maker, the polymeric polyethylene generally consumes a substantial quantity of the nickel catalyst component present since the resulting product is typically a solid agglomeration of polyethylene and nickel particles.
From the foregoing it can be seen that it would be highly desirable if an alternative means could be devised for preparing and introducing the nickel complex catalyst into the oligomerization reactor which would minimize the formation of undesirable polymeric polyethylene and the equipment plugging and fouling problems associated therewith.